Since the visual system of humans is complex and concerns many aspects to judge the differences between two images, i.e., the textures and flow inside the images, methods which pursue absolutely similarity differences (PSNR/SSIM) will not always perform well. Although distortion measures have been widely used, the improvement in reconstruction accuracy is not always accompanied by an improvement in visual quality. In fact, researchers have shown that the distortion and perceptual quality are at odds with each other in some cases [1]}. The image perceptual quality of an image \(\hat{x}\)  is defined as the degree to which it looks like a natural image, which has nothing to do with its similarity to any reference image.
